For the accurate control of a photographic photoprocessing machine, it is necessary to replenish the processing baths to compensate for consumption of the chemicals therein as the photographic material is processed, and thus to maintain the chemical activity of the processing solutions. Improvements in the formulations of the processing solutions, and a desire to reduce the volume of liquid effluent which is produced, have led in recent years to a gradual reduction in the rate and amount of replenishment required. This, in turn, has led to a requirement for increasingly accurate control of the replenishment process. For black and white materials, the volume of replenishment solution required for the developer and the fixer stages of the processing is a function of the area of the material processed and of the amount of developed image on the material. The width of the sheet may be determined in a number of ways. In many graphic arts, that is to say high contrast black and white, processing machines, for example, where a variety of material widths may be used, the area is usually approximately determined by measuring the sheet width and the length by means of microswitches extending across the width of the entrance to the processor that are activated by the passing material. The width is determined by the number of microswitches that are activated, and the length by the time for which the microswitches are activated multiplied by the transport speed of the processor.
Another method of obtaining the width and length, and thus the image area, is to have the image exposing apparatus, for example, an image setter, transmit this information to the processor, as is the case, for example with the Linotype Hell Hercules PRO/Advantage.TM. image setting system. The processor is then able to use the information from the image setter to enable accurate replenishment and thus to maintain good process control.
U.S. Pat. No. 4,506,969 (Pako Corporation) discloses a film width and transmittance scanner system of a graphic arts film processor in which the transmittance of light through the film is measured along a line or set of parallel lines positioned at a skew angle with respect to the direction of film travel. U.S. Pat. No. 3,554,109 (Logetronics Inc) discloses an image monitoring and control system for determining the optical densities developed in sheets of image-bearing photosensitive material, for controlling the feeding of replenishment chemicals to a film processor.
A preferred method of measuring the width of an image bearing sheet of photographic material, however, is disclosed in our contemporaneously-filed patent application (GB 9828439.1, filed Dec. 24, 1998) in which the width, and the transmittance or reflectance, of a sheet of photographic material is determined by apparatus employing a plurality of optical sensor arrangements. The entire disclosure of this application is incorporated herein by this reference.
With reference to the photographic application of the invention, in a conventional photoprocessor, the sheet of photographic material is driven therethrough by rollers. The spacing from one roller, or set of rollers, to the next is arranged to be less than the length of the smallest sheet to be handled by the processor. Thus, the speed of a sheet through the processor can be controlled by means of a drive roller to be at a constant and known (or at least easily calculated) value. The length of the sheet can then easily be derived, for example, using microswitches as mentioned above. The processing apparatus will also have associated therewith an optical scanning apparatus for determining the integrated film transmittance, or reflectance, which, together with the film length and width is then used to control the replenishment of the film processor chemical solutions. The scanner may, in principle, be positioned anywhere in the processor, for example after the fixing stage. However, if the scanner is provided as a stand alone piece of apparatus and receives the photographic sheet subsequently to the sheet leaving the speed-controlled photoprocessor rollers, then the sheet will move freely, that is to say under its own weight, and accelerate from the photoprocessor as it passes through the scanning apparatus. In a preferred method of operation the scanning apparatus samples the light transmitted through the sheet at regular intervals. During the period of movement at constant speed, therefore, when the sheet is being driven through the scanning apparatus by the processor's drive roller, each transmittance sample value obtained will be associated with a constant area of material, equal to the width of the sheet multiplied by the distance moved by the sheet between samples. Towards the end of the passage of the sheet, however, measurements made whilst the sheet is accelerating should have a variable and increasing area of the sheet associated with them. It will be appreciated that errors will arise unless appropriate correction is made.
It is one object of the present invention to provide an inexpensive yet accurate method for measuring the transmittance, or reflectance, of a freely-moving sheet of photographic material.